This invention relates to a color picture tube using a shadow mask as the color selective electrode, and more particularly, to a shadow mask support member designed to support the shadow mask.
In a conventional color picture tube using a shadow mask, the shadow mask is heated to expansion during the operation due to the impact thereupon of the electron beams. This causes a variation in path of electron beams defined by the aperture of the shadow mask and allows the electron beams to pass therethrough to reach the fluorescent screen. As a result, a poor color purity occurs. For the purpose of preventing the occurrence of such a poor color purity, the shadow mask support member supporting the shadow mask so as to permit the same to be disposed approximately to the fluorescent screen inside the envelope can be constructed so that the shadow mask with an increase in the temperature thereof may be placed in proximity to the fluorescent screen, thereby compensating for the path variation of electron beams.
As a means for achieving such a compensation, two shadow mask support members are known, one of which is of such a type as disclosed in the specification of U.S. Pat. No. 3,803,436 wherein a bimetal prepared by laminating a metal plate of low-thermal expansion upon a metal plate of high-thermal expansion is used as a constituent part of the shadow mask support member, and the other of which is of such a type as disclosed in the specification of U.S. Pat. No. 3,573,527 wherein a bimetal prepared by joining a low-thermal expansion metal plate to a high-thermal expansion metal plate in side by side joined relationship to each other is used as the shadow mask support member.
Any of such known shadow mask support members is welded at one end to a mask frame arranged to hold the shadow mask in place, and is engaged directly or indirectly at the other end with a panel pin built in the inner face of the envelope. In this case, in order to secure the fixation of the shadow mask support member to the mask frame, both are welded together at a plurality of spots.
Each of the above-mentioned known shadow mask support members sufficiently performs the function of preventing the mislanding, due to a temperature rise occurring during the operation of the color picture tube, of the electron beams onto the fluorescent material radiating a particular color. It has turned out, however, that the known shadow mask support member degrades the characteristic of the color picture tube, as below, by undergoing a thermal treatment at high temperature in the process of manufacturing the color picture tube.
That is, in the manufacturing process of the color picture tube, a thermal treatment is carried out during, for example, an evacuation step at as high a temperature as 400.degree..about.450.degree. C. which is extremely high as compared with the temperature of 70.degree..about.80.degree. C. prevailing when the color picture tube is in operation. As a result, any of the shadow mask, mask frame and shadow mask support member is heated to expansion during said evacuation step. On the other hand, the shadow mask support member is partly or wholely constituted by the bimetal, and the metal constituting this bimetal differs from the mask frame in respect of the thermal expansion coefficient. Owing to this difference, thermal deformation occurs in the joining portion between the shadow mask support member and the mask frame. That is, since the shadow mask support member is required to support the heavy shadow mask assembly, it is spot-welded to the side wall of the mask frame at several spots. As a result, a thermal stress, owing to the difference in thermal expansion coefficient between the two, acts on the welding spots. This thermal stress becomes very high due to the thermal treatment at high temperature. Thus, it becomes possible that a permanent deformation is made in the mask frame or the shadow mask support member. Consequently, the position of the shadow mask relative to the fluorescent screen is often displaced, resulting in a lower color reproducibility of the color picture tube. Therefore, for reducing the effect of such a thermal deformation, it is necessary to perform an additional treatment of passing the intermediate product through a stabilizing oven for thermal stabilization of the same.